How to have different materials assigned to same mesh gradate into each other (without using a mapping node)
You can use vertex colors. If you paint a mask in black and white in Vertex Paint mode(ctrl+tab -> Vertex Paint) and then mix your desired shaders or textures using vertex color node, the textures will blend gradually:



You can make various gradients from a lot of things with nodes so this is probably not the only solution. I would recommend not to limit yourself by excluding usage of any nodes such as mapping node when working with your materials for no reason. It's usually best to use all the tools available to you.